Automated Mineralogy As a Tool for Geolocation Based on Soil Trace

ORE Open Research Exeter TITLE Automated mineralogical proﬁling of soils as an indicator of local bedrock lithology: a tool for predictive forensic geolocation AUTHORS Pirrie, D; Crean, DE; Pidduck, AJ; et al. JOURNAL Geological Society, London, Special Publications DEPOSITED IN ORE 11 December 2019 This version available at http://hdl.handle.net/10871/40081 COPYRIGHT AND REUSE Open Research Exeter makes this work available in accordance with publisher policies. A NOTE ON VERSIONS The version presented here may diﬀer from the published version. If citing, you are advised to consult the published version for pagination, volume/issue and date of publication OFFICIAL 1 Automated mineralogical profiling of soils as an indicator of local 2 bedrock lithology: a tool for predictive forensic geolocation 3 4 Duncan Pirriea,d, Daniel E. Creanb, Allan J. Pidduckb, Timothy M. Nichollsb, 5 Roy P. Awberyb, Robin K. Shailc 6 7 8 aHelford Geoscience LLP, Trelowarren Mill Barn, Mawgan, Helston, Cornwall, 9 TR12 6AE, UK 10 11 bAWE Plc, Aldermaston, Reading, RG7 4PR, UK 12 13 cCamborne School of Mines, University of Exeter, Penryn Campus, Penryn, 14 Cornwall, TR10 9FE, UK 15 16 dSchool of Applied Sciences, Faculty of Computing, Engineering and 17 Sciences, University of South Wales, Glyntaf, Pontypridd, Rhondda Cynon 18 Taff, CF37 4AT, UK 19 20 ABSTRACT 21 22 The use of soil evidence to identify an unknown location is a powerful tool to 23 determine the provenance of an item in an investigation. We are particularly 24 interested in the use of these indicators in nuclear forensic cases, whereby 25 identification of locations associated with for example, a smuggled nuclear 26 material, may be used to indicate the provenance of a find. The use of soil 27 evidence to identify an unknown location relies on understanding and 28 predicting how soils vary in composition depending on their geological / 29 geographical setting. In this study, compositional links between the 30 mineralogy of forty soils and the underlying bedrock geology were 31 established. The soil samples were collected from locations with broadly 32 similar climate and land use across a range of geological settings in a ‘test 33 bed’ 3500 km2 area of South West England. In this region, the soils formed 34 through chemical weathering of the bedrock, representing a worst case for © British Crown Owned Copyright 2018/AWE 1 OFFICIAL 35 this type of forensic geolocation due to the high degree of alteration of the 36 parent rock during soil formation. The mineralogy was quantified using 37 automated SEM-EDX analysis. The soil mineralogy and texture are consistent 38 with the underlying geology as indicated by regional-scale geological 39 mapping. 40 41 Keywords: soil forensics, provenancing. 42 43 Introduction 44 45 It is widely recognised that soil is a useful class of trace evidence which 46 can be used to test a possible association between a geographical location 47 and material recovered from, for example, footwear, clothing, vehicles and 48 objects used during an offence (e.g. Bull et al., 2006; Pirrie and Ruffell, 2012). 49 In general, most published forensic soil science studies have addressed the 50 use of soil as an evidential tool, testing an association between an object and 51 a known location through comparative analysis (Fitzpatrick et al., 2009). 52 However, soil analysis can also be used to attempt to identify unknown 53 locations based on the soil characteristics (Bowen and Craven, 2013; Lark 54 and Rawlins, 2008; Owens et al., 2016). This use of soil trace evidence to 55 determine the geographic position of otherwise unknown locations can be 56 referred to as predictive geolocation (Pirrie et al., 2017) and is of increasing 57 interest to the forensic geoscience community (e.g. Stern et al., 2019; Caritat 58 et al., 2019). 59 One aspect of our interest in predictive geolocation relates to its 60 potential to determine the geographic source (provenance) of an item in a 61 nuclear forensic investigation (Mayer et al., 2012). These scenarios involve 62 the discovery or interdiction of nuclear material outside regulatory or 63 institutional control, such as so-called ‘nuclear smuggling’ cases (Wallenius et 64 al., 2006; Mayer et al., 2015). In these investigations, there may be a wide 65 range of potential sources of the material, (e.g. facilities which have 66 processed the particular type of material), and therefore the potential to 67 constrain the possible origins of a find through predictive geolocation analysis 68 would be of great value in establishing the provenance of a found material. © British Crown Owned Copyright 2018/AWE 2 OFFICIAL 69 This is complementary to the types of analysis which fall under the field of 70 nuclear forensic science (Keegan et al., 2016), which relate to the properties 71 of the material itself, such as the identification of particular processes through 72 the evaluation of signature chemical properties of the material. 73 Predictive geolocation analysis relies on signatures accumulated from 74 the environments to which a nuclear material has been exposed during 75 production, transport and storage; both before and after institutional control 76 was lost. Packing and repackaging of materials may introduce a number of 77 internal interfaces where materials from different environments may 78 accumulate. These environmental signatures may include natural materials 79 such as soils and dusts, to which forensic soil geolocation analysis techniques 80 can be applied to indicate location(s) associated with the sample. 81 For this predictive geolocation analysis to be practical, it is necessary 82 to select key parameters which can be measured in a realistic (short) 83 timescale for use in an investigation, and which have a high degree of 84 specificity in indicating an unknown location. Owing to the wide availability of 85 spatial geological reference datasets (e.g. from national geological surveys), 86 indicators related to the underlying lithology at a location are useful in this 87 context (Pirrie et al., 2013). Determination of the likely source geology from 88 soil analysis would allow an investigation to be focussed to areas where this 89 setting is known to occur. For this approach to have forensic validity, the 90 relationship between soil mineralogy and underlying geology has to be 91 established. Soil-forming processes can profoundly alter the composition of 92 the parent material depending on environmental conditions, and therefore 93 there is a need to understand the specificity with which lithological signatures 94 could be detected in a range of soil forming environments. 95 In this paper we present a systematic study of the relationship between 96 the underlying geology and soil mineralogy in a geologically varied 3500 km2 97 area of SW England. The area provides a ‘test bed’ with a variety of well 98 documented major rock types and settings. Soils within the area are mainly 99 mature loams, developed through chemical weathering and human 100 management. This represents an advanced state of alteration of the 101 underlying rock and therefore a high potential for disturbance of the 102 mineralogical signatures linking the soil to the associated bedrock. Automated © British Crown Owned Copyright 2018/AWE 3 OFFICIAL 103 mineralogy based on scanning electron microscopy was used as a rapid 104 method for characterisation the mineralogy and texture of the soils. This tests 105 the link between rapidly measureable soil characteristics (in this case 106 mineralogy) and widely available geological datasets, such that an indication 107 of potential geological settings could be provided from an unknown soil 108 sample in an investigation. 109 110 Overview of Study Area 111 112 In this study 35 geographical locations over an area of approximately 113 3500 km2 in Cornwall and Devon, SW England, UK were selected for soil 114 sampling (Fig. 1, Table 1). Factors such as variations in climate and land-use 115 were minimised as far as possible, such that the dataset allows the 116 significance of the underlying bedrock geology as the main control on soil 117 mineralogy to be tested. All of the sampling locations would also have had 118 surface superficial sediments present mantling the bedrock geology to 119 variable extents. 120 The selected sampling area whilst geologically varied, shows the 121 repetition of a number of major rock types, and samples from these were 122 taken to test discrimination between soils developed on different age units 123 composed of the same dominant rock types. SW England has also been a 124 recent focus for baseline environmental and geochemical surveys based on 125 both airborne datasets and regional surface soil and sediment sampling 126 programmes (Beamish, 2015; Kirkwood et al., 2016). 127 128 Geology 129 130 SW England is an Upper Palaeozoic massif whose bedrock geology, in 131 common with much of western Europe, was strongly influenced by the late 132 Palaeozoic Variscan mountain-building episode (Shail and Leveridge, 2009). 133 The near-surface geology is dominated by Devonian and Carboniferous 134 metasedimentary rocks (Fig 1.). Other significant features include associated 135 minor basic intrusive / extrusive igneous rocks, a major granite batholith which 136 was emplaced in the Early Permian, and the Lizard Complex, which includes © British Crown Owned Copyright 2018/AWE 4 OFFICIAL 137 partially serpentinised mantle peridotites, gabbros and high-grade 138 metamorphosed mafic igneous rocks. Devonian and

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